As is well known, most commercial color printing uses three or four primary colour inks, typically: Cyan (C), Magenta (M), Yellow (Y) and usually also Black (K). A color image to be printed is first separated into a respective layer (colour plane) for each ink and each of these layers is then printed, one on top of the other, using a halftoning process.
The need for halftoning arises from the fact that most existing printing devices are bi-level, that is, they are only capable of printing solid ink on a media substrate or leaving the media unprinted, and are unable to produce intermediate ink tones. With halftoning the original continuous tone image (or each of its color planes) is transformed into a distribution of small dots whose size or number varies depending on the tone level to be produced. When printed this gives to the eye (looking from a normal viewing distance) an illusion of a full range of intermediate tone levels even though the printing device is only bi-level.
Many halftoning methods are currently in use; however, most of the high and medium quality image printing is done using a halftoning method with clustered-dot elements. In this method each of the color planes of the original continuous tone image is transformed into a regular screen of equidistant dots where the size of the screen dots varies according to the image tone level but the frequency and angle of the screen remains fixed; this is known as AM (amplitude modulated) halftoning. Each such halftone screen is characterized by three parameters: the screen angle (the direction in which the screen dots are aligned); the screen frequency (the number of screen dots per inch or centimeter in the direction of the screen); and the dot shape.
As a result of interference between halftone screens printed in superposition, macrostructures in the form of one or more moiré patterns may appear and a careful choice of the screen angles and frequencies is required to minimize moiré visibility.
In one screen combination commonly used for color printing, the screens all have the same frequency and the screen of the black ink, which is the most prominent color, is set to 45 degrees with the cyan and magenta screens being set to 45±30 degrees, namely 15 and 75 degrees. These angles differences of 30 degrees between the superposed screens are large enough to make the strong moiré between the first harmonics of each pair of layers practically disappear. The fourth screen, belonging to the yellow ink to which the eye is much less sensitive, is placed at 15 degrees from two of the other screens (usually at 0 degrees). This commonly-used screen combination is very sensitive to small angle or frequency deviations so that any slight misalignment will result in a moiré becoming visible.
The superposition of dot screens, as well as giving rise to the potential for moiré-pattern macrostructures, also produces microstructures not present in the original screens. These microstructures, known as ‘rosettes’, are two to five times the periods of the screens and are local groupings of the superposed screen dots. It is the variation in the form of the rosettes across the superposed screens, due to the screen frequencies and angles, that produce the macrostructure moirés. Even with constant tone images (uniform dots), changes in form of the rosettes across the superposed screens, for whatever reason, will gives rise to changes in both the reflected OD (Optical Density) and perceived color.
As a consequence, not only is it highly desirable to use moiré-free screen geometries, but in order to keep consistent colors within the page and among pages, printing devices need to maintain consistent registration among the color planes. Registration changes between color planes result in a different overlap which, in turn, leads to a different form of rosette and thus a change in reflected OD color. Registration changes may arise for a variety of reasons such as mechanical features of the printer, mechanical shocks, deformations of the printing substrate (typically paper), etc. Failure to maintain consistent registration can result in visible bands on the printed page with a changed color and OD, as well as different colors across the page and between pages.